1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, and more particularly to an evaporative emission control system, which prevents evaporative fuel generated in the fuel tank from being emitted into the atmosphere by controlling pressure within th e fuel tank to a negative value during operation of the engine as well as during stoppage of the same.
2. Prior Art
Conventional evaporative emission control systems for internal combustion engines for vehicles are generally constructed such that to prevent evaporative fuel generated in the fuel tank from being emitted into the atmosphere, the fuel tank is connected via a canister to the intake system of the engine so that evaporative fuel generated in the fuel tank is absorbed by the canister during stoppage of the engine and desorbed from the canister to be supplied to the engine for combustion during operation of the engine.
Further, there has already been proposed an improved evaporative emission control system of this kind, (for example, in U.S. patent application Ser. No. 09/021,004, assigned to the assignee of the present application,) which negatively pressurizes the interior of the fuel tank during operation of the engine so as to hold the fuel tank under negative pressure not only during operation of the engine but also during stoppage of the same, to thereby prevent evaporative fuel within the fuel tank from being emitted into the atmosphere, even if a filler cap of the fuel tank is removed for refueling.
The proposed system includes a temperature sensor which detects the temperature of fuel within the fuel tank, and a tank internal pressure sensor which detects the pressure within the fuel tank (hereinafter referred to as "the tank internal pressure"), to set the desired pressure value within the fuel tank to an excessively negative value, i.e. a lower value than the actually required value according to the temperature of fuel within the fuel tank, in view of an expected increase in the tank internal pressure Pt. Further, the proposed system includes a control valve arranged in an evaporative fuel passage extending between the fuel tank and the intake system of the engine, for controlling a flow rate of evaporative fuel supplied from the fuel tank to the intake system due to negative pressure within the intake system during operation of the engine. The opening of the control valve is feedback-controlled in response to an output from the tank internal pressure sensor such that the tank internal pressure becomes equal to the desired pressure value. Thus, the tank internal pressure is normally controlled to and held at the desired pressure value.
In the proposed system, however, the negative pressurization of the fuel tank to the desired pressure value is normally carried out during traveling of the vehicle to utilize negative pressure within the intake system of the engine developed during operation of the engine. As a result, when the control valve is opened to start the negative pressurization of the fuel tank, evaporative fuel within the fuel tank is drawn into the intake system to cause a sudden change in the air-fuel ratio of a mixture supplied to the intake system, whereby a shock is generated to degrade drivability and exhaust emission characteristics of the engine.
On the other hand, to avoid degradation of drivability and exhaust emission characteristics of the engine due to the negative pressurization of the fuel tank, a limit value is provided for the flow rate of evaporative fuel to be supplied from the fuel tank to the engine intake system for negative pressurization of the fuel tank. As shown in FIG. 1, the limit value is set, for example, as shown in FIG. 1, to a larger value (liter/min) as at least one of the engine rotational speed and the intake system absolute pressure is higher. The limit value for the flow rate of evaporative fuel for negative pressurization of the fuel tank can limit the upper limit of the negative pressurization rate of the fuel tank.
Even though the limit value is provided, however, if the flow rate of evaporative fuel for negative pressurization is set to the limit value immediately upon the start of the negative pressurization of the fuel tank, a shock can be generated due to a sudden change in the air-fuel ratio of the mixture, resulting in the above-mentioned inconvenience.
On the other hand, when the tank internal pressure is controlled to the desired pressure value, the tank internal pressure approaches the desired value with the lapse of time. During the control, however, when the difference between the intake system pressure and the tank internal pressure becomes smaller, the flow rate of evaporative fuel drawn from the fuel tank into the intake system lowers, and hence the negative pressurization rate of the fuel tank lowers. FIG. 2 shows a change in the tank internal pressure with the lapse of time during the negative pressurization of the fuel tank. As is clear from the figure, since the negative pressurization rate is lowered with the lapse of time, the interior of the fuel tank cannot be negatively pressurized to the desired pressure value in a short time, especially when the vehicle has traveled only over a short distance after refueling. This makes it difficult to always maintain the interior of the fuel tank under negative pressure during operation of the engine as well as during stoppage of the same.
Thus, the proposed system has a problem of contradictory requirements, i.e. restraint of the negative pressurization rate of the fuel tank and increase of the same.